1. Field of the Invention
The present invention generally relates to an optical pickup apparatus that reproduces signals recorded on an optical disk, which is an optical recording medium for recording signals, such as video information. More particularly, the present invention is concerned with an optical pickup apparatus which reproduces signals recorded on an optical disk provided with a plurality of signal pit strings in each of which a plurality of pits are arranged in the radial direction of the optical disk.
2. Description of the Related Art
It is desirable that information be more rapidly reproduced as an increased amount of information is recorded on the optical disk. Particularly, it is necessary to use a broadband recording and reproducing system capable of recording and reproducing high-frequency signals in order to record and reproduce information used in a high definition television system on and from optical disks. In order to realize such a broadband recording and reproducing system, it may be attempted to more finely record information on the optical disk, and read an increased number of pits per unit time, or per unit length.
However, in such an attempt, the fine recording of information needs a shorter minimum pit length on the optical disk. In order to reproduce thus recorded pit images at a required resolution level, it becomes necessary to broaden a spatial frequency band of the optical pickup apparatus used for reproduction. For this requirement, it is necessary to shorten the wavelength of the light source or increase the numerical aperture of the objective lens.
The currently available semiconductor laser devices cannot emit light having such a required wavelength. Thus, it becomes necessary in this case to use a gas laser or a laser using a non-linear optical element. Consequently, the cost and size of the optical pickup apparatus get increased. On the other hand, an increased numerical aperture needs a flatter optical disk or an optical disk having a uniform disk surface. However, it is very difficult to produce such optical disks. As a result, it is very difficult to broaden the spatial frequency band for reproduction.
It may be possible to rotate the optical disk at a higher speed in order to reproduce signals in a high-frequency band. However, it is necessary to use a more powerful motor for rotating the optical disk. In addition, it is difficult to control the focusing position following an increased revolution of the motor.
The optical pickup apparatus may include a lateral single mode semiconductor laser, and a photodetector. In this case, the semiconductor laser serves as a point light source, which has a light emitting point having a diameter of approximately 0.1 .mu.m, for example. A half mirror may be disposed in the optical path of the semiconductor laser to separate a projection light directed to an optical disk and a reflected light therefrom from each other, while an objective lens is provided to focus the projection light onto the optical disk, and collect the reflected light therefrom. The photodetector is positioned to detect the reflected light. In such a construction, the laser beam emitted from the semiconductor laser is reflected by the half mirror, and forms an image on a recording plane of the optical disk by means of the objective lens. The wavelength .gamma. of the semiconductor laser and the numerical aperture NA are selected such that .gamma./NA is greater than 0.1 .mu.m. Hence, the size of the spot on the optical disk is limited to .gamma./NA due to a diffraction limit.
As has been described previously, information is recorded on the optical disk in the form of pits, which cause optical change. By way of example, the pits formed on the recording plane cause a change in the refractive index. The recording of information is performed by sensitizing pigment contained in a recording film or changing the state of the recording film to a crystal state or an amorphous state. In the above manners, areas having different refractive index values are formed on the recording plane. The reflected light from the above recording plane has different intensity levels due to the differences in the refractive index. Then, the reflected light is converged by the objective lens. Some portion of the reflected light passes through the half mirror. At this time, the half mirror may be adapted to cause the reflected light to have an astigmatic characteristic. That is, the reflected light from the half mirror has different converting positions in the longitudinal and lateral directions of the light axis. The photodetector is located between the different converging positions.
It is necessary to reduce the distance between adjacent signal tracks in order to increase the signal recording density in the optical pickup apparatus as described above. As the distance between adjacent signal tracks reduces, the light spot formed on a signal track is more affected by a change in the reflectance of adjacent signal tracks. Hence, leakage of light from the adjacent signal tracks takes place, and the S/N ratio of the reproduced signal is degraded.
Japanese Patent Application Laid-Open Publication No. 57-58,248 discloses a high-density reading method, in which a plurality of light sources are used. Three adjacent signal tracks are projected at once by these light sources, and far-field patterns respectively formed by the light sources are detected by photodetectors. The output signals of the photodetectors corresponding to the respective far-field patterns are mutually subtracted from each other in accordance with a leak rate measured beforehand. Using the results of the above calculation, the quantity of leakage can be reduced. However, the above method needs a light spot having a size corresponding to the diffraction limit. If the interval between the adjacent signal tracks is smaller than the limited size of the light spot, the influence of the outer signal track will increase significantly. As a result, this reading method does not bring about great improvement.
As described above, it is very difficult to improve the information recording density on the optical disk by narrowing the distance between adjacent signal tracks.